As has already been said, its all about the direction of rotation of the motor. If you look from the pinion end, the motor spins anti-clockwise. Therefore putting the motor in the right hand side of a belt driven chassis would result in the spur (and therefore the drivetrain) rotating backwards.
With a shaft car, you could mount the motor on either side of the car. However, if you put the motor in the left hand side of a shaft car, the pinion end would need to point forwards to achieve the correct drivetrain rotation direction (as the motor always spins anti-clockwise). Alternatively you could put it in the left hand side pointing backwards, and put the diff crown gears on the opposite sides of the bevel gear in the gearbox housings.
As far as I'm concerned torque steer has nothing to do with it; you'd get the same amount of torque steer with any of the shaft layouts described above. The reason the tc3, pro 4 etc... had the motor in the right hand side pointing backwards will have been purely for design purposes, and making the best use of the space available.
Thats why most cars are belt driven now; it eliminates torque steer as all the rotation is in the forwards/backwards direction. A shaft drive car will always try and twist under acceleration/braking due to the inertia created by the motor and drivetrain.
Shaft drive cars are actually more efficient, and are probably a better option for low power motors which dont cause torque steer problems.